The present invention relates to a method and apparatus for making thick-walled, flat molded plastic articles, in particular plastic discs.
There is an increasing tendency, in particular in the automobile industry, to substitute glass panes with plastic panes because plastic panes are lighter in weight and shatter-resistant. However, conventional manufacturing processes are unable to make plastic panes with adequate and reproducible surface quality so that their use in the automobile industry has been limited to sun roofs but has not been viable enough for use as a side window, rear window, or windshield.
Optical surfaces of small-area items with even wall thickness are normally produced, using an injection molding process, whereby a plastic mass is introduced during the filling stage into a mold cavity via small passages. As amorphous plastics exhibit a high reduction in thickness (material shrinkage of about 10 to 20%) during the cool-down phase, the material shrinkage is compensated during a subsequent holding pressure phase through addition of plastic melt to the injection piston of the injection molding machine. However, such a process leads to inhomogeneities so that its application for large-area articles is unsuitable.
In a so-called standard injection compression process, plastic mass is supplied in a first filling phase into a pre-enlarged cavity, and subsequently compressed by closing the mold. Such an injection compression process is appropriate for simple optical items such as lenses because the compression prevents the formation of sink marks as a result of material shrinkage. However, large-area plastic items, such as slabs of several square meters, cannot be reproducibly made because of the difficulty to maintain a precise enough compression stroke. The automobile industry, for example, requires precision of less than tenth of millimeter which cannot be reproducibly implemented by the machines involved here, which apply clamping forces exceeding 2000 tons.
European patent publication no. EP-A-0144622 or U.S. Pat. No. 4,828,769 describes further variations of injection compression processes in which material pressed out from the cavity during the compression, or the compression phase is started already during the injection phase.
International publication no. WO 98/10417 describes a process of making very thin-walled plastic articles, such as compact discs. In this process, plastic mass is injected into a mold in such a way that the mold opens before the mold is fully filled.
International publication no. WO 98/53975 describes a mold with an elastically supported core which shifts during injection of plastic mass to subsequently compensate shrinkage losses. The elastic support results in a floating core system that is incapable of realizing a precisely defined wall thickness of the end product.
It would therefore be desirable and advantageous to provide an improved method and apparatus for making flat molded plastic articles to obviate prior art shortcomings and to be useful in making large-area plastic articles with good optical properties.